Present day leg extension machines all work on the same primary principle of fixing the positions of the operator's thighs (femur bones) while applying resistive forces to the operator's lower legs through circular paths about the operator's knee joints which tend to make the legs flex. The quadriceps muscles of the thighs are developed as they oppose these forces by tending to make the legs extend during the exercise movement.
It is a well known fact and easily verified that due to joint mechanics, angles of pull of muscles, physiological make-up of muscles, etc. that in a leg extension movement the least amount of force can be generated in the fully extended position while the most force can be generated in the range of 60-90 before the fully extended position. Consequently, a leg extension machine which varies the resistive force applied to correspond with the positionally related strength capabilities of the operator's quadriceps muscles as they extend the legs will be more effective at developing those muscles.
The standard method of varying the resistive force applied on present day leg extension machines is through the use of cams used in conjunction with chains or cables, all of which have inherent problems. The problem with cams is that they are relatively hard to manufacture. The problem with cables is that, because of their relatively small cross sectional area, they carry very high tensile stresses (a 1/8" cable carrying 200 lbs, for example, has a tensile stress in it of approximately 16,300 psi). These already high stresses are multiplied and become cyclic (introducing fatigue wear) when a cable moves along bending over a small diameter pulley. These high cyclic stresses, applied to relatively small cross sectional areas, make cables stretch (eventually decreasing the machine's intended range of motion) and eventually fray and wear out (leading to replacement or catastrophic failure). Chains, while not suffering the fatigue wear that cables do, are subject to stretching at their many joints (thus decreasing the machine's intended range of motion). They, also like cables, are subject to relatively high tensile stresses and in addition are noisy and introduce spurious drag to the machine.
In view of the advantages of applying a variably resistive force to an operator's leg extending muscles in a leg extension exercise movement, and the disadvantages of obtaining such a force through the use of cams, chains, or cables it is the objective of the disclosed invention to introduce a leg extension exercise machine which applies a variably resistive force to an operator's leg extending muscles through the use of a force-varying mechanism which uses only rigid members and pinned joints, thereby eliminating the problems associated with force-varying mechanisms using cams, chains, or cables.